220 39184 <99477e9d-42e6-42f7-87ae-747b70d7e5c8@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: "T. C." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Concepts requires clause should allow types that constexpr implicitly convert to bool? Date: Mon, 16 Jul 2018 15:50:25 -0700 (PDT) Lines: 203 Approved: news@gmane.org Message-ID: <99477e9d-42e6-42f7-87ae-747b70d7e5c8@isocpp.org> References: <79029827-71df-4af1-ae50-607e96c5e322@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_167514_1341363581.1531781425142" X-Trace: blaine.gmane.org 1531781300 20831 195.159.176.226 (16 Jul 2018 22:48:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 16 Jul 2018 22:48:20 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCQ43G7NQIIRBMWCWTNAKGQEYMW5LUA@isocpp.org Tue Jul 17 00:48:16 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f197.google.com ([209.85.161.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ffCHw-0005J0-9u for gclcip-std-proposals@m.gmane.org; Tue, 17 Jul 2018 00:48:16 +0200 Original-Received: by mail-yw0-f197.google.com with SMTP id u1-v6sf37316516ywg.6 for ; Mon, 16 Jul 2018 15:50:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=e513lPmnWPwm7zeLXJ7Kn3JqoJcLWOSLdr5t8fHeP/0=; b=uZC/mimT3NWhmq7JiWDvpAptegQ+gT1A0QKGqqu2f/P0VS3Te8DqRwUjpqprEZ/cwV CaJS+dzpODk7CA9HHSr8DhGOMs8bSxF0LR8Npzp4dGcQzD9XQWtQFL60i8lS3zc8z5ln KOGxbdIfejhD36XB1tuIN3wnN/XSDo9BidEY066hsIU9T0/ogBoORxjMNAlHeJwtAZHw opeXcRWn30YNRVJlNBCfVL2xZrtzGFu86jRTtuSophAj03EFhgF21Jx98tcgHjN7geSJ 7zlDYDxeQg2QeLMwnkcVCnFB0Wqiy4VvmEGXk5KcURZfaDG9gzXI6s5j0MEHA2IgfZgr gZ/g== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=e513lPmnWPwm7zeLXJ7Kn3JqoJcLWOSLdr5t8fHeP/0=; b=CyoTLXo/yQqBVU3f5fB3lAmwNib+9VSpZ/CjbjYdISO2QgKYBmXvi9zQCgn+tRZdca DWNDWOYzlIHpCtm0ovFhvbMbVfF/FPDx84HAV8t3CJ9LrYBz5HywgiXE1mS0txttUzy1 L9Lzzgs7ydKX7O63oH8RF8jFpl9wW6WoR3seLrCUG3zU/J8M4es7pZ8M1iEVKJFusuhO bzCIUzWtxxIHIXCRVSUYYqKRuQ5NnfIjymQuckUShT37oIGse9cAiZ+Q3oln/h9ffIQS JqjN6PJNLM4eJem+cGgECFquCP69fdGRnzQ/ixNGQTYes7bHNspZq8cN57obF7lXcKBu akcg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=e513lPmnWPwm7zeLXJ7Kn3JqoJcLWOSLdr5t8fHeP/0=; b=PDT0Fz1z3dV827YaCB5aSpHQTdWlRCVdIURkQExU9vWkmxF2S2CEG4YhFEFCAO8dUw 0usVrYdkMxXhPv6FAxbZdxSuQZwjDQhnSOWyDlCyjEyCJWBWLj/xVn2MOlJ4uppWOJau N1+ysweBNN+QTIpATBfnIIlxaYu/pf2TA4wkUAfFba5X5iuIrZSdd++WGHL6CXZAeqDw Q2cVuN8s6lt0pqPJPmeKOs2FzGirpo+/p+ZUWd3nmon25wL+I4eEOapamQnBFrI4asJR W7/Z1zlSI0hreKjpMAGHDIVRkR3CsD3Vg/I0KHIC0jSG1AR0Q9N45zfkU/ndjuO9gOaj z9Kg== X-Gm-Message-State: AOUpUlEfA9UVCArear03tcF9Wb8iFyFxGcKCsOiuIpgTcujVa5j+8mke Gt2QFkiBJmPBYL+3NI6OPiIF0Q== X-Google-Smtp-Source: AAOMgpc0Zh9LavLBOPOkr8vpXu18VkMavwNxtsd+1wqZTuW67grSTCEYLI+c0M2Y2gObMBH7woWG1Q== X-Received: by 2002:a25:9d92:: with SMTP id v18-v6mr5743545ybp.31.1531781427031; Mon, 16 Jul 2018 15:50:27 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a81:9482:: with SMTP id l124-v6ls4297503ywg.20.gmail; Mon, 16 Jul 2018 15:50:25 -0700 (PDT) X-Received: by 2002:a81:7dd6:: with SMTP id y205-v6mr2214009ywc.1.1531781425693; Mon, 16 Jul 2018 15:50:25 -0700 (PDT) In-Reply-To: X-Original-Sender: rs2740@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:39184 Archived-At: ------=_Part_167514_1341363581.1531781425142 Content-Type: multipart/alternative; boundary="----=_Part_167515_1174510557.1531781425143" ------=_Part_167515_1174510557.1531781425143 Content-Type: text/plain; charset="UTF-8" The "exactly bool" requirement was introduced during the Skillman concepts meeting. Permitting implicit conversions opens up a can of worms: struct C { constexpr operator int() const { return 0; } constexpr explicit operator bool() const { return true; } }; constexpr bool b1 = C{}, b2(C{}); static_assert(!b1); static_assert(b2); template concept C1 = C{}; // contextual conversion, or implicit conversion? template concept C2 = C{} && true; // contextual? template requires (C{}) // contextual or implicit? void f(); template requires (C{}) && true // contextual? void g(); template requires (C{}) // there's an implicit logical AND in the constraint here - is the conversion contextual or implicit? void h() requires true; On Sunday, July 15, 2018 at 5:38:44 AM UTC-4, Nicolas Lesser wrote: > > Yes, this is intended. See [temp.constr.atomic]p3. > > And I agree that any type implicitly convertible to bool should be allowed. > > On Sun, Jul 15, 2018, 2:31 AM > wrote: > >> Apologies, if this is off-topic. >> >> It seems that the following doesn't compile in all versions of gcc that I >> tried (7.2, 8.1, 9.0). It complains that the type of the value in the >> "requires" clause is not bool, although the value is implicitly constexpr >> convertible to bool. >> >> >> template >> constexpr auto is_even (std::integral_constant = {}) >> { >> return std::bool_constant<(v % 2 == 0)> {}; >> } >> >> template >> requires is_even (std::integral_constant {}) // (*) >> int some_function () >> { >> return (v * 3); >> } >> >> int main () >> { >> std::cout << some_function () << '\n'; >> } >> >> >> Workarounds are, of course, to append "()" or to prepend "!!" to the line >> marked with (*), but might be better without? If this is not a bug in gcc, >> I hoped someone would be able to make that change easily at this point. >> >> -- >> You received this message because you are subscribed to the Google Groups >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to std-proposal...@isocpp.org . >> To post to this group, send email to std-pr...@isocpp.org . >> To view this discussion on the web visit >> https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/79029827-71df-4af1-ae50-607e96c5e322%40isocpp.org >> >> . >> >> -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/99477e9d-42e6-42f7-87ae-747b70d7e5c8%40isocpp.org. ------=_Part_167515_1174510557.1531781425143 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
The "exactly bool" requirement was introduced du= ring the Skillman concepts meeting. Permitting implicit conversions opens u= p a can of worms:

struct C { constexpr operator int() co= nst { return 0; } constexpr explicit operator bool() const { return true; }= };

constexpr bool b1 =3D C{}, b2(C{});
= static_assert(!b1);
static_assert(b2);

t= emplate<class>
concept C1 =3D C{}; // contextual conversion= , or implicit conversion?

template<cl= ass>
concept C2 =3D C{} && true; // contextual?
<= /div>

template<class>
=C2=A0 =C2=A0requi= res (C{}) // contextual or implicit?
void f();

template<class>
=C2=A0 =C2=A0requires (C{})= && true // contextual?
void g();

template<class>
=C2=A0 =C2=A0requires= (C{})=C2=A0 // there's an implicit logical AND in the constraint here = - is the conversion contextual or implicit?
void h() requires tru= e;=C2=A0

On Sunday, July 15, 2018 at 5:38:44= AM UTC-4, Nicolas Lesser wrote:
Yes, this is intended. See [temp.constr.atomic]p3.=

And I agree that any ty= pe implicitly convertible to bool should be allowed.

On Sun, Jul 1= 5, 2018, 2:31 AM <toby....@gmail.com> wrote:
Apologies, if this is off-topic.

It seems that the following doesn't compile in all= versions of gcc that I tried (7.2, 8.1, 9.0). It complains that the type o= f the value in the "requires" clause is not bool, although the va= lue is implicitly constexpr convertible to bool.

<= br>
template<typename T, T v>
constexpr auto is_even= (std::integral_constant<T, v> =3D {})
{
=C2=A0 return std::bool_constant<(v % 2 =3D=3D 0)> {};<= /div>
}

template<typename T, T v>
=C2=A0 requires is_even (std::integral_constant= <T, v> {})=C2=A0 // (*)
int some_function ()
{
=C2=A0 = return (v * 3);
}

= int main ()
{
=C2=A0 std::cout << some_function<int, 4> () <&l= t; '\n';
}

Workarounds are, of course, to = append "()" or to prepend "!!" to the line marked with = (*), but might be better without? If this is not a bug in gcc, I hoped some= one would be able to make that change easily at this point.

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposal...@isocpp.org.
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To view this discussion on the web visit https://groups.google.com= /a/isocpp.org/d/msgid/std-proposals/79029827-71df-4af1-ae50-= 607e96c5e322%40isocpp.org.

--
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